Simple lateral flow immunoassay devices have been developed and commercialised for detection of analytes in fluid samples, see for example EP291194. Such devices typically comprise a porous carrier comprising a dried mobilisable labelled binding reagent capable of binding to the analyte in question, and an immobilised binding reagent also capable of binding to the analyte provided at a detection zone downstream from the labelled binding reagent. Detection of the immobilised labelled binding at the detection zone provides an indication of the presence of analyte in the sample. The assay device may additionally incorporate a control zone to indicate that the test has been carried out satisfactorily. The control zone is typically positioned downstream from the detection zone and may comprise an immobilised binding reagent for a labelled binding reagent.
EP1484613 discloses an electronic assay device for the determination of an analyte comprising an optical detection means wherein said assay measures signals from an analyte measurement zone over time with respect to one or more thresholds and provides a result to the user.
A sandwich immunoassay is often the immunoassay assay of choice when detecting analytes. However, a sandwich assay is not always possible, for example in the case of small molecules such as haptens which may not be large enough to allow the simultaneous binding thereto of two different binding partners. A dose-response curve prepared using a typical lateral flow device employing a sandwich immunoassay shows increasing levels of signal with increasing analyte up to the point where at higher analyte levels the curve tends to plateau. At yet higher analyte levels, the signal begins to decrease due to preferential capture at the detection zone of analyte which has not yet bound to labelled reagent. This phenomenon is known as the hook effect. Thus sandwich immunoassays exhibit a limited assay range due to the fact that the signal amount or intensity observed at higher analyte levels may be the same, or even less, than that observed at lower analyte levels.
Thus in order to detect analytes at higher concentrations, especially in other than a qualitative manner, it is necessary to employ alternative methods or assay devices.
WO2005052716 discloses an assay device for the detection of an analyte at high concentration levels wherein the assay device comprises a scavenging zone comprising capture reagent capable of binding to the analyte. This has the effect of preventing some of the analyte from being detected at a detection zone and lowers the sensitivity of the assay.